In a typical electrically controlled trailer braking system, the towing vehicle or car includes an electrical power source such as a battery which is used to power all of the electrical components of the car. The towed vehicle or trailer includes a solenoid actuated braking mechanism on each of the trailer wheels. When a trailer is mechanically coupled to a car to provide a tandem towing arrangement, a separate means is provided for electrically coupling the car storage battery to the trailer brake solenoids. The brake pedal or a separate control means of the towing car operates a variable resistor or voltage divider which controls the amount of electrical current flowing from the car battery to the trailer brake solenoids and thereby controls the trailer braking effort. Thus, assuming the trailer and car systems are electrically coupled, as the car operator applies the brakes of the towing vehicle, the trailer brakes will also be applied.
As is shown in U.S. Pat. No. 3,840,276 Jubenville, it is known to provide a safety or warning light which will indicate that a supplementary portion of a vehicle braking system is operative. However, Jubenville relates to an electrical trailer braking system adapted to control oscillations or swaying of the trailer relative to the towing vehicle. Jubenville's warning light relates only to the oscillation controlling part of his braking system and does not indicate whether the basic trailer braking system is operative. Nor does Jubenville provide means for selectively testing the operability of portions of the basic trailer braking system.